Juniata County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 23,509. Its county seat is Mifflintown. The county was created on March 2, 1831, from part of Mifflin County and named for the Juniata River. The county is part of the Central Pennsylvania region of the state. Mountains in Juniata County include Tuscarora Mountain and Shade Mountain. Agricultural land and forested land make up most of the county's area.